(a) Technical Field of Invention
This invention relates to a coating composition and, in particular, to a composition for priming a polymeric substrate.
(b) Background of the Art
The problems of achieving effective bonding between the respective layers of a multi-laminar polymeric assembly are well known, and are encountered in relation to a variety of end-use applications. For example, the production of a photographic film may involve the bonding of a light-sensitive emulsion to a relatively unreceptive hydrophobic substrate such as a film of a linear polyester resin. Similarly, it may be necessary to adhere a layer of a photopolymerisable resin to a base sheet in the production of a relief printing plate. It has therefore become established practice to provide an intermediate anchor coating or primer layer on a polymeric substrate to promote the adhesion thereto of a subsequently applied layer or layers.
(c) The Prior Art
European patent publication EP-A-156532 relates to the production of an inkable recording sheet, suitable for use with an ink jet printer, and discloses that an ink-absorbent resin layer may be bonded to a self-supporting polymeric film substrate by a priming medium which comprises a swelling agent and, preferably, a partially hydrolysed vinyl chloride-vinyl acetate copolymer.
A similar priming medium is disclosed in European patent publication EP-A-101210 as being suitable for bonding a resin layer to a polymeric substrate in an assembly adapted to receive a photopolymerisable material in the production of a relief printing plate.
Although, in general, such a priming medium has proved to be eminently satisfactory in the specified products, a problem of relatively inferior adhesive bonding has been observed between a polymeric, especially a polyester, substrate and a limited class of resin layers applied thereto. Applied resin layers which are susceptible to poor bonding include cellulosics, such as hydroxyethylcellulose and cellulose acetatephthalate, and polymers of acrylic and/or methacrylic acid.
We have now devised a coating composition which, although of general applicability, is particularly suitable for use as a priming medium with the aformentioned resins.